Carbon, water, and heat flux responses to experimental burning and drought in a tallgrass prairie

Drought and fire are common disturbances to grassland ecosystems. We report two years of eddy covariance ecosystem–atmosphere fluxes and biometric variables measured in nearby burned and unburned pastures in the US Southern Great Plains. Over the course of the experiment, annual precipitation (∼600 mm yr⁻¹) was lower than the long term mean (∼860 mm yr⁻¹). Soil moisture decreased from productive conditions in March 2005 dry, unproductive conditions during the growing season starting in March 2006. Just prior to the burn in early March 2005, burned and unburned pastures contained 520 ± 60 and 360 ± 40 g C m⁻² of total above ground biomass (AGB) and litter, respectively. The fire removed approximately 200 g C m⁻² of litter and biomass. In the 2005 growing season following the burn, maximum green AGB was 450 ± 60 and 270 ± 40 g C m⁻², with corresponding cumulative annual net ecosystem carbon exchange (NEE) of −330 and −150 g C m⁻² for the burned and unburned pastures, respectively. In contrast to NEE, cumulative mean sensible heat and water fluxes were approximately equal in both pastures during the growing season, suggesting either an increase in water use efficiency or a decrease in evaporation in the burned relative to the unburned pasture. In the 2006 growing season, dry conditions decreased carbon uptake and latent heat, and increased sensible heat fluxes. Peak AGB was reduced to 210 ± 30 g C m⁻² and 140 ± 30 g C m⁻² in the burned and unburned pastures, respectively, while NEE was near zero. These results suggest that the lack of precipitation was responsible for most of the interannual variation in carbon exchange for these un-irrigated prairie pastures.